Precision electronic parts formed on a substrate such as a semiconductor wafer and a liquid crystal display board are sequentially subjected to various manufacturing processes and test processes such as photoresist application, thin film deposition, formation of an oxide film and a nitrification film, etching, and heat treatment, and are commercialized through a series of various processing steps. A substrate, a surface of which has been subjected to one processing step, is stored in a sealable clean container called FOUP (Front-Opening Unified Pod), and conveyed to the next step by conveying means in a factory. However, due to a difference in time required for the processing of each processing step, the standby time is required until the next processing step. At this time, the substrate is temporarily stored in a storage device called a stocker, and waits until the next processing starts. In the semiconductor manufacturing process, when a new processing step is introduced, a wafer for test called a test wafer is used for determining processing parameters to be actually used, in order to optimize the processing step.
In the related art, a standby semiconductor wafer or test wafer is stored in the FOUP and put on a shelf installed in a zone in which the degree of cleanliness is relatively high in a clean room. Recently, however, in order to minimize the footprint of a stocker in a semiconductor manufacturing factory, a wafer stocker has been required, instead of the FOUP, for storing a wafer. The wafer stocker is a container dedicated for storage, and can store wafers while minimizing the intervals between the wafers.
In Patent Literature 1, space saving is achieved in such a manner that storage rings, on which wafers are placed, are accumulated at intervals so as not to bring the wafer surfaces into contact with each other. Moreover, the upper surface and the bottom surface of each accumulated storage ring are covered with a cover, and a region for storing each wafer is isolated from an external environment. It is thus possible to maintain the inside of the region at an inert gas atmosphere such as nitrogen. The conveyance of the wafer into/out of each storage ring is performed by dedicated conveying means installed in the stocker. At the time of conveyance, a space (opening) accessible by the conveying means is formed by a drive mechanism (referred to as “opening means” below) that lifts each storage ring by a predetermined amount individually.
The wafer storage amount in a unit space has increased by the above-mentioned structure, but the following new problems have occurred. When a wafer stored in an accumulated storage ring is carried out or a wafer is carried into the storage ring, the storage ring to be accessed and a storage ring below it are lifted by the opening means up to a height at which a gripper of conveying means can be inserted. Then, when the access of the gripper ends, the lifted storage rings are returned to the original accumulation positions by the opening means. In this case, however, the following trouble occurs. That is, when the storage rings are lifted, the lifted storage rings and adjacent rings collide with each other, generating dust which contaminates wafers placed on the storage rings.
In addition, when a target storage ring and all the storage rings disposed above it are lifted by the opening means, the capacity of a wafer storage space formed by each storage ring and the upper and lower covers temporarily increases, and an atmosphere including dust is sucked into the storage space from an external environment. This prior art includes means for substituting an inert gas atmosphere for the inside of the storage space. However, since the opening area at the time of conveyance is large, the gas filled in the storage space diffuses outside in a short time, making it necessary to supply a large amount of inert gas every time the opening operation is performed. In addition, a natural oxide film is formed on a semiconductor wafer under processing by oxygen and vapor included in the air entering from the external environment, making the following processing incomplete and reducing a yield.